First, gtk+ is for C, while qt is for C++. Another major difference is that qt is more than just a widget toolkit, but an application runtime environment that provides widgets. This means that qt provides string handling, database connectivity, etc., although you don't have to use anything but the widgets and application objects, if you wish.
I thing maemo is mostly written in C, so some parts will probably have to be rewritten in C++.
This article may help a bit, although it only compares qt with gtkmm (the c++ bindings to gtk):
http://www.telegraph-road.org/writings/why.html
This article should be taken with a grain of salt, as it's pretty old, and may be inaccurate today.
I started using gtk+ with python, way back in the 1.x versions. The 2.x bindings for python were much better, allowing me to write more pythonic code using gtk+.
Later on, I decided to try out qt3, and I haven't looked back since. While it took a bit of getting used to, I found that it was easier to use qt, rather than gtk+, although I'm hard pressed to figure out exactly why.
One of the things I liked about qt over gtk+ was the separation of the layout widgets and the interactive widgets. Coming from gtk, this was something that took me a while to understand, but once I got the hang of it, I liked it, and think that it's a better way to organize the widgets. With gtk, a vbox holds child widgets, such as buttons, labels, etc. So if you want to rearrange them in an hbox, you have to destroy those widgets and make new ones in the hbox. In qt, the layout widgets are of type "layout", and you can only have layout children in layout widgets. The interactive widgets are children of the main widget (or a child widget of the main widget). These widgets are "placed" into the layout, but can be removed without being destroyed, allowing you to rearrange the layout more easily.
I also prefer the signal/slot mechanism in qt over the callback mechanism in gtk. On the average, it makes it easier to glue your widgets together, but there are a few circumstances where a callback mechanism is preferred, in which case you have to invent a new signal(s) and chain them together. This is because there is no order of slots called when a signal is emitted.
Also, the qt documentation was better, more organized, and easier to read than the gtk docs (at least around the time I switched ~2004).
Probably the largest reason why we're even having this discussion is due to licensing. Gtk gained a lot of popularity, due qt being licensed under the trolltech license, which restricted developers from using the free version in commercial products. The switch to gpl didn't do much to change this, although you could then create commercial products, but you also had to release the source for those products. So if you wanted to keep the source closed and use qt, you still had to purchase a commercial qt license.